In analyzing the timing of an internal combustion engine, determination is made of the number of degrees of rotation (timing angle) between the instant a piston in a selected cylinder reaches its so-called "top dead center" position and the instant that combustion takes place in that cylinder. Automobile manufacturers commonly specify the timing angle at a specific engine speed as a compromise to numerous considerations which must be taken into account, such as reducing emissions, maximizing gas mileage, optimizing performance, etc. For example, an automobile manufacturer may specify that combustion should start five degrees after the piston has reached top dead center at an engine speed of 1,250 rpm.
A timing/tachometer apparatus must identify the events of top dead center and combustion. The top dead center event is identified in the following manner. An internal combustion engine includes a wheel mounted on the end of the engine crank shaft so as to rotate therewith. A notch, projection or other marking means is located at a point in the periphery of the wheel. A receptacle for a magnetic sensor is carried by the engine block and is so located that the notch on the rotating wheel will pass the receptacle a known number of degrees of crank shaft rotation after the number one cylinder has reached it top-dead-center position. In presently manufactured diesel engines, such known number or "offset angle" is either 20.degree. or 9.5.degree., while in current spark-ignited engines, the offset angle is one of a number of possibilities: 9.5.degree., 10.degree., 20.degree., 52.5.degree., 68.degree., 135.degree. or 190.degree.. The magnetic sensor, which is part of the timing/tachometer apparatus, has a transducer to provide an electrical signal when the cylinder is at top dead center plus the offset angle.
In a diesel engine, the combustion event may be sensed by light occurring during combustion. In a diesel engine, fuel is first delivered to a prechamber. To increase the temperature of the prechamber to a value to insure engine starting, a so-called "glow plug" is threaded into an access port of each such prechamber. The glow plug is energized a few seconds before starting; but, while the engine is actually operating, the glow plug is not energized. The glow plug for the prechamber associated with the cylinder being analyzed is removed and a luminosity probe inserted in its place. The luminosity probe includes a sensor which responds to light produced during the combustion event and generates an electrical signal.
In a spark-ignited engine, the combustion event is identified by sensing energization of the spark plug associated with the number one cylinder. A clip having a transformer therein is applied to the conductor connected between the distribution and that spark plug. An electrical signal is magnetically generated in the clip.
The electrical signal produced in response to the top-dead-center event and the electrical signal in response to the combustion event are applied to apparatus which determines and displays the timing angle between the two. The serviceman timing the engine can make adjustments in order that the displayed number matches the timing angle specified by the manufacturer for a particular speed.
Another important parameter which must be determined is engine speed. A timing meter usually has a tachometer associated with it. Some currently available timing/tach meters are capable of analyzing a diesel engine. Others are capable of analyzing a spark-ignited engine, thereby necessitating twice the number of pieces of equipment a service station must purchase and maintain.
Most current timing/tach meters are of the analog type so that the timing angle is indicated by the instantaneous location of a pivoting pointer. A pointer's position is more difficult to read than a digital display. Although there are some timing/tach meters that display timing angle and speed digitally, they are not as accurate as is required under certain circumstances. Moreover, the numbers on such digital displays change rapidly (referred to as "bobbling") making it more difficult to read. Analog tachometers do not have sufficient range on one scale to display engine speed through the entire range of speeds which the tachometer must evaluate. Instead, a knob must be adjusted to change ranges.
Analog tachometers are very difficult to use for checking cranking engine speeds, (the speed of the engine as it is being started) of between 150 and 200 rpm. The quivering of the pointer is especially severe during cranking, making it more difficult to read.
As above explained, the notch or other marking means located in the flywheel is related to top dead center of the number one cylinder. Therefore, a combustion event in the number one cylinder will supply information on the timing angle of that cylinder. There are circumstances when it is important to evaluate timing in one or more of the other cylinders. Currently available timing meters do not accommodate that because the amount of offset angle provided by the timing meter is fixed. In a few instances, the offset angle can be selected from one or two values. To evaluate timing of cylinders other than the number one cylinder requires the capability of programming other offset angles into the timing meter.
Certain presently available tachometers enable determination of spark-ignited engine speed by using signals from the inductive probe, and other tachometers enable determination of such engine speed by using signals from the magnetic probe. There are certain instances in which it is desirable to use one and in other instances in which it is desirable to use the other. In current diesel-engine tachometers, engine speed is determined by the use of the magnetic pickup probe.
Some automobiles have two notches or projections on the flywheel. Conventional timing/tach meters are not able satisfactorily to analyze timing because of an inability to distinguish between the two.